Background
The selection of a material for a particular structural application depends on its mechanical properties. There are a number of standard tests that measure properties specifically designed to yield required engineering design data. For example, one of the most convenient types of strength tests to analyze is the tensile test where the specimen is loaded in uniform uniaxial tension. Tension tests are also conducted using ancillary equipment and instruments at a range of temperatures, such as cryogenic, ambient and above. From such tests, various mechanical properties of metallic materials needed for design applications can usually be gleaned without too much difficulty, except for several specialized tests mentioned below. However, attempts to determine the desired mechanical properties of materials that behave in a brittle manner without precautionary measures to reduce the applied load misalignments will result in extreme errors. This is because materials that behave in a brittle manner do not realign any small inherent eccentricity and the resulting bending stress will not only be amplified but also will be superimposed on to the axial stress during testing. Thus, the error will be extreme (this is mathematically shown in Error Estimates presented below), and premature failure will occur.
The effects of misalignment on the stresses and strains measured in studies of fracture strength of materials in a brittle state, not only in the testing of tensile specimens but also compression, and torsion specimens, as well as combinations thereof; and including cyclic fatigue specimens, need to be mentioned. These effects can also be present in ductile metallic materials when testing for stress-rupture life, creep, fatigue life, plastic microstrain, alloy strengthening, surface-sensitive strength, fracture of notched-tensile specimens and, precracked-notched-tensile specimens.
Most of the following section, except for some minor alterations, was taken directly from ASTM Designation: E 1012-89, "Standard Practice for Verification of Specimen Alignment Under Tensile Loading."